It has long been desirable to employ high speed centrifugal pumps as fuel pumps in aircraft employing turbine engines. Fuels used in aircraft turbine engines are typically of low lubricity and centrifugal pumps are ideally suited for pumping such liquids without excessive wear, particularly if the fuel itself may be used as a lubricant thereby eliminating circuits for lubricating oil. As can be imagined, when a lubricating oil circuit is avoided, pump weight, volume, and complexity are all reduced.
In many cases, for a given pumping capacity at rated engine speed, a centrifugal pump will occupy a considerably lesser volume than a typical positive displacement pump used for the same purpose. Usually, this reduction in volume will translate into a weight savings as well.
As the reduction of the size of the envelope occupied by a given component is reduced, the aircraft designer is provided with greater flexibility in achieving an aerodynamically slippery design. Consequently, a smaller envelope made possible by reduced size of a component raises the potential for more efficient operation of aircraft through the reduction in drag.
At the same time, the accompanying weight reduction enables aircraft range to be increased. The weight carrying capability of the aircraft heretofore devoted to transporting a positive displacement fuel pump can, in part, be used to increase fuel carrying capacity and/or other pay loads.
Notwithstanding the foregoing, centrifugal pumps as fuel pumps in turbine powered aircraft have not yet received an appreciable degree of utilization for the purpose. Conventional centrifugal volute pumps do not have the ability to provide fuel flow at high pressure at low engine speeds, particularly during engine starting sequences. Moreover, many high performance aircraft today include components, such as nozzles, that are altered during engine operation to achieve a change in performance. These alterations are conventionally achieved hydraulically and it is not unusual for pressures of the hydraulic fluid to be at a level of approximately 2500 psig. While this can be obtained through the use of hydraulic circuits using conventional hydraulic fluids, to eliminate complexity and for other obvious reasons, modern day aircraft employ fuel at high pressure as the hydraulic fluid since it is continually being consumed by the engine and therefore is not subject to coking as would be the case with conventional hydraulic oils in separate hydraulic circuits.
As one might well imagine, when changes in engine geometry are required to change aircraft performance, virtually immediate response to a command for geometry alteration is required. This cannot be achieved using conventional centrifugal volute pumps operating at engine speed with the consequence that other means must be employed, adding complexity, weight and component volume to the aircraft.
The present invention is directed to overcoming one or more of the above problems.